Memory loss has long been recognized as a common accompaniment of aging. The inabilities to recall the name of a recent acquaintance or the contents of a short shopping list are familiar experiences for everyone, and this experience seems to become more common as we age.
Over the last few decades, the medical community has changed its view of memory loss in the elderly. These problems were viewed in the past as inevitable accompaniments of aging, often referred to as “senility” or “senior moments.” More recently, physicians have shifted their view of memory loss, such that memory impairment of a certain degree is now considered pathological, and thus indicative of a disease process affecting the brain. The threshold most physicians use to make this judgment is that memory loss has progressed to such an extent that normal independent function is impossible; for instance, if one can no longer successfully manage one's own finances or provide for one's own basic needs. This degree of cognitive impairment has come to be referred to as dementia.
However, many older individuals may complain of memory problems, but still manage to independently accomplish all their customary tasks. Usually, their ability to function well is based on compensation for these difficulties, such as increased reliance on a calendar or on reminder notes, lists, etc. In some cases, these memory difficulties are a sign that worsening memory loss is on the horizon.
The syndrome of subjective memory problems has come to be commonly known as “Mild Cognitive Impairment” (MCI), although other terms have been used, including “Cognitive Impairment, Not Dementia” (CIND). The patient with MCI complains of difficulty with memory. Typically, the complaints include trouble remembering the names of people they met recently, trouble remembering the flow of a conversation, and an increased tendency to misplace things, or similar problems. In many cases, the individual will be quite aware of these difficulties and will compensate with increased reliance on notes and calendars. Most importantly, the diagnosis of MCI relies on the fact that the individual is able to perform all their usual activities successfully, without more assistance from others than they previously needed.
Several studies have examined the cognitive performance of patients with MCI. These have demonstrated that, in general, these patients perform relatively poorly on formal tests of memory, even when compared with other individuals in their age group. They also show mild difficulties in other areas of thinking, such as naming objects or people (coming up with the names of things) and complex planning tasks. These problems are similar, but less severe, than the neuropsychological findings associated with Alzheimer's disease. Careful questioning has also revealed that, in some cases, mild difficulties with daily activities, such as performing hobbies, are evident.
Several studies have demonstrated that memory complaints in the elderly are associated with a higher-than-normal risk of developing dementia in the future. Most commonly, the type of dementia that patients with MCI are at risk to develop is Alzheimer's disease, though other dementias, such as Vascular Dementia or Frontotemporal Dementia may occur as well. However, it is also clear that some patients with these complaints never develop dementia.
Certain features are associated with a higher likelihood of progression. These include confirmation of memory difficulties by a knowledgeable informant (such as a spouse, child, or close friend), poor performance on objective memory testing, and any changes in the ability to perform daily tasks, such as hobbies or finances, handling emergencies, or attending to one's personal hygiene.
One factor that had to be controlled for in many of these studies was depression, as many patients with depression also complain about their memory. Several studies have suggested that certain measurements of atrophy (shrinkage) or decreased metabolism on images of the brain (PET or MRI scans) increase the chances of developing dementia in the future.
Although these above factors increase the chances of going on to develop dementia, it is not possible currently to predict with certainty which patients with MCI will or will not go on to develop dementia. Thus, many of these measures, particularly the measurements from brain images, are still considered to be useful only for research.
One neurological disorder that is most widely known for its progressive loss of intellectual capacities is Alzheimer's disease (AD). Worldwide, about 20 million people suffer from Alzheimer's disease. AD is clinically characterized by the initial loss of memory, followed by disorientation, impairment of judgment and reasoning, which is commonly referred to as cognitive impairment, and ultimately by full dementia. AD patients finally lapse into a severely debilitated, immobile state between four and twelve years after onset of the disease.
The key pathological evidence for AD is the presence of extracellular amyloid plaques and intracellular tau tangles in the brain, which are associated with neuronal degeneration (Ritchie and Lovestone (2002)). The extracellular amyloid plaques are believed to result from an increase in the insoluble amyloid beta peptide 1-42 produced by the metabolism of amyloid-beta precursor protein (APP). Following β, γ secretion, these amyloid beta 1-42 peptides form amyloid fibrils more readily than the amyloid beta 1-40 peptides, which are predominantly produced in healthy people. It appears that the amyloid beta peptide is on top of the neurotoxic cascade: experiments show that amyloid beta fibrils, when injected into the brains of P301 L tau transgenic mice, enhance the formation of neurofibrillary tangles (Götz et al. (2001)). In fact, a variety of amyloid beta peptides have been identified as amyloid beta peptides 1-42, 1-40, 1-39, 1-38, 1-37, which can be found in plaques and are often seen in cerebral spinal fluid.
The amyloid beta peptides are generated (or processed) from the membrane anchored APP, after cleavage by beta secretase and gamma secretase at position 671 and 711 or 713, respectively. In addition, high activity of beta secretase results in a shift of the cleavage at position 1 to position 11. Cleavage of amyloid-beta precursor protein by alpha secretase activity will generate Aβ 1-17 and gamma secretase activity at 40 or 42 generates the non-pathological p3 peptide. Beta secretase was identified as the membrane anchored aspartyl protease BACE, while gamma secretase is a protein complex comprising presenilin 1 (PS1) or presenilin 2 (PS2), nicastrin, Anterior Pharynx Defective 1 (APH1) and Presenilin Enhancer 2 (PEN2). Of these proteins, the presenilins are widely thought to constitute the catalytic activity of the gamma secretase, while the other components play a role in the maturation and localization of the complex. The identity of the alpha secretase is still illustrious, although some results point towards the proteases ADAM 10 and TACE, which could have redundant functions.
A small fraction of AD cases (mostly early onset AD) are caused by autosomal dominant mutations in the genes encoding presenilin 1 and 2 (PS1; PS2) and the amyloid-beta precursor protein (APP), and it has been shown that mutations in APP, PS1 and PS2 alter the metabolism of amyloid-beta precursor protein leading to such increased levels of amyloid beta 1-42 produced in the brain. Although no mutations in PS1, PS2 and amyloid-beta precursor protein have been identified in late onset AD patients, the pathological characteristics are highly similar to the early onset AD patients. These increased levels of amyloid beta peptide could originate progressively with age from disturbed amyloid-beta precursor protein processing (e.g. high cholesterol levels enhance amyloid beta peptide production) or from decreased amyloid beta peptide catabolism. Therefore, it is generally accepted that AD in late onset AD patients is also caused by aberrant increased amyloid peptide levels in the brains. The level of these amyloid beta peptides, and more particularly amyloid-beta peptide 1-42, is increased in Alzheimer patients compared to the levels of these peptides in healthy persons.